1. Field of the Invention
This invention relates to refrigeration devices such as refrigerant compressors, refrigerators and refrigerating air conditioners which use a refrigerant containing a hydrofluorocarbon as a main ingredient.
2. Description of the Conventional Art
Conventional refrigeration units use chlorine-containing flons (e.g. chloro-difluorocarbon CFC12) as refrigerants, and mineral oils or synthesis oils which are soluble with the refrigerant, such as alkyl benzene oil, as lubricating oil. Recently, however, it has been discovered that flon refrigerants containing chlorine destroy the ozone layer. Therefore, flons containing chlorine are being replaced by flons containing hydrogen which will not affect the environment, such as hydrofluorocarbon HFC134a. One example of a conventional refrigeration unit in which the hydrofluorocarbon HFC134a is used as a refrigerant is shown in FIG. 8.
As disclosed in the publication "Tribologist" vol. 35, No. 9 (1990), pp 621-626, where a refrigeration unit is provided using a hydrofluorocarbon refrigerant, namely, HFC134a refrigerant, the mutual solubility of the refrigerant and the refrigerating machine oil is an important factor. That is, since mineral oils or synthesis oils which are used as a lubricating oils in conventional refrigeration devices using CFC12 refrigerant are insoluble with the HFC134a refrigerant, these mineral oils and synthesis oils can not be used in refrigeration units in which HFC134a refrigerant is used. Therefore, PAG (polyoxyalkyleneglycol) or ester group refrigerating machine oils which are soluble with HFC134a refrigerant are employed. FIG. 8 shows a refrigeration unit which uses an HFC134a refrigerant. In FIG. 8, reference numeral 1 designates a compressor for compressing a refrigerant gas; 2, a condenser for condensing a high pressure refrigerant gas discharged from the compressor 1; 3, a capillary tube; 4, an evaporator; 5, an accumulator which adjusts the amount of refrigerant; and 6, refrigerating machine oil pooled in compressor 1 for lubrication of the slide parts of the compressor 1 and for sealing of the compressor chamber. The refrigerating machine oil 6 is PAG 6a or an ester group refrigerating machine oil 6b.
The behavior of the oil in the refrigeration unit will now be described. The refrigerant compressed by compressor 1 is supplied to condenser 2. In the case of a compressor with a high pressure container, although a large part of lubricating oil 6 which is used for sealing the compression chamber is separated from the refrigerant, the lubricating oil 6 which is of the order of 0.5 to 1.0% by weight of the refrigerant is discharged from the compressor together with the refrigerant. The lubricating oil 6 thus discharged, being soluble with the refrigerant, is high in fluidity and is therefore returned through condenser 2, capillary tube 3, evaporator 4 and header 5 to compressor 1. Thus, the lubricating oil 6 is present in compressor 1 at all times; that is, the lubrication is carried out suitably at all times. Furthermore, even when compressor 1 is not operated for long periods of time; that is, even when the refrigerant is held still, its foaming can be prevented.
Conventional refrigeration units using HFC134a refrigerant are constructed as described above. The PAG 6a oil has a volume resistivity of 10.sup.7 to 10.sup.10 .OMEGA..cm, and a saturated water content of about 2500 ppm; and the ester group refrigerating machine oil has a resistivity and water content of 10.sup.12 to 10.sup.14 .OMEGA..cm and about 1500 ppm, respectively. Both oils are much lower in electrical insulation and in hygroscopicity than refrigerating machine oils used with CFC12 refrigerant (which has an about 10.sup.15 .OMEGA..cm volume resistivity and about 500 ppm saturated water content). The electrical insulation greatly affects the long range reliability of the compressor. In order to suitably handle the components of the compressor, or the compressor itself, the saturated water content should as low as possible.
Furthermore, in assembling a refrigerator, production quality suffers from a number of problems in handling the machine oil; for instance, it is necessary to shorten the period of time for which the refrigeration circulation system is open. When a large quantity of water gets in the refrigeration circulation system, it tends to form sludge, or freeze in the circulation system and clog up the capillary tube, thus impairing the refrigerating action.
Furthermore, if, in conventional refrigeration units using HFC134a refrigerant, the oil is high in hygroscopicity it will suffer from the following difficulties: that is, it is rather difficult to prevent the components of the compressor from rusting and ice may be formed in the capillary tube and the expansion valve of the refrigeration unit, clogging them up. Hydrolysis of the ester oil is accelerated by the water content, resulting in formation of sludge. In addition, hydrolysis of any polyethylene-terephthalate material employed as insulation material for the motor is accelerated, thus resulting in the further formation of sludge. In order to eliminate the above-described difficulties in the manufacture of a refrigeration unit using HFC134a it is necessary to remove water from the machine oil and the refrigerant circuit more carefully than for a refrigeration unit using CFC12 refrigerant. That is, it is necessary to increase the water removing ability of a dryer in the refrigerant circuit. For this purpose, it is necessary to use a dryer having a larger capacity.
Moreover, conventional refrigerating units using HFC134a refrigerant are disadvantageous in the following point: when the compressor is stopped, the liquid-phase refrigerant is returned to the compressor through its suction inlet. When it is started again, the lubricating oil in the compressor is led into the refrigerant circular system together with the liquid-phase refrigerant. Since the HFC134a refrigerant is low in solubility with the lubricating oil, it is difficult for the lubricating oil thus led to return into the compressor until its flow rate reaches a certain value. That is, the lubricating oil is not sufficiently supplied to the compressor, so that the compressor may break down.